A known process for producing alloy semiconductor crystals is known as the Vertical Bridgman method, wherein an ampoule containing a solid charge is displaced in a furnace that has a temperature gradient. As the temperature of the furnace is increased, the charge is heated and then melts. The ampoule is then translated in the furnace such that the charge is directionally solidified. Alternatively, the ampoule may be stationary, and the furnace or the temperature field in the furnace may be translated relative to the ampoule resulting in directional solidification of the charge. Through judicious selection of the furnace temperature profile and translation rate, the solidified charge may be in the form of one or more single crystal grains. Vertical Bridgman is one of a number of directional solidification techniques for production of semiconductors contained in an ampoule; other techniques include Horizontal Bridgman, and Vertical or Horizontal Gradient Freeze.
The properties of semiconductor crystals produced by the standard directional solidification techniques are influenced by a number of innate features of the process. For example, as the solidification process is initiated from a fixed volume of the charge, any segregation of the melt constituents results in the enrichment or depletion of the melt from that constituent, which in turn results in axial variation of the concentration of that constituent in the solidified charge. Another feature of the standard Vertical Bridgman technique is the influence of containment on the quality of the grown material. The contact between the solidification interface and the ampoule wall results in generation of crystalline defects, such as dislocations. Spurious nucleation at the ampoule wall may also interfere with growth of large single crystal grains.
Elemental and alloy semiconductors produced by directional solidification all exhibit, to a varying extent, the issues described above. Directional solidification of materials that have a high vapor pressure is further complicated by the partial evaporation of the volatile material into the unavoidable open-space in the ampoule, which results in a deviation of the melt composition from desired stoichiometric conditions. In the case of alloys containing cadmium, such as CdTe and CdZnTe, changes in the melt stoichiometry due to a very small loss of cadmium can result in production of significant amount of tellurium precipitates in the grown material.